No standardized susceptibility testing procedures exist for rapidly- growing mycobacteria or Nocardia species. Several different procedures have been suggested, including broth microdilution and the E-test (manufactured by AB Biodisk, Culver City, CA). We are comparing a microdilution procedure (JustOne, manufactured by Radiometer America, Inc., Westlake, OM) with the E-test procedure for their comparability and reproducibility. We are simultaneously looking at a number of other variables, such as inoculum concentration, incubation time, and testing medium. The JustOne strips are useful because they allow selection of dilutions of individual antimicrobial agents that may be particularly relevant clinically, rather than the use of a whole array of agents as found in the usual microdilution plate format. We hope to be able to select a procedure that will provide rapid and reproducible results, and not be so technically demanding as to be unsuitable for use in the routine diagnostic laboratory. Work to date indicates that the E-test produces results that are difficult to interpret, and we intend to concentrate our further efforts on the microdilution procedure. We are also involved in a multi-institution collaborative study to determine the inter- and intra- institution reproducibility of both the E-test and a microdilution procedure for susceptibility testing of rapidly-growing mycobacteria.